Hardenable steel heat treatment



Patented Oct. 20, 1942 UNITED STATES PATENT OFFICE HARDENABLE STEEL HEAT TREATMENT Harold J. Elmendorf, West Boylston, Mala,-

signor to United States Steel Corporation of Delaware, a corporation of Delaware No Drawing. Application May 8, 1941,

Serial No. 392,533

3 Claims. (Cl. 148-21) This invention is a hardenable steel heat treatment, it being understood that such steel is any containing suflicient carbon to make it hardenable in a commercial sense and that it may contain other alloying elements, this application being a formation of austenite to a structure other than coarse pearllte or martensite, the cooling being with such speed that the steel arrives at this temperature while it retains all or practically all of its austenite, it then being held at this temperature for a sufficient time to permit all its austenite to directly transform to the structure characteristic of this transformation temperature. The advantages of this treatment are known and need no prolonged discussion beyond mentioning that it produces a product that is free from microscopic cracks and which has a higher ductility and impact strength for the hardness obtained than is normally attainable by the older quench and temper methods. I

The above treatment has the disadvantage that the time required for the austenite to transform is considered too long for some applications, the object of the present invention mainly being to provide a treatment involving the advantages of the above described treatment to a practical degree and yet which provides for a shorter austenite transforming period. 7 g

This object is attained byheating the steel to transform its structure to austenite, quenching the steel to a temperature causing its austenite to directly transform to a structure other than coarse pearllte or martensite, this quenching being suiliciently rapid so that the steel arrives at the described temperature while retaining all or most of its austenite, holding the steel at this temperature until only a portion of its austenite transforms to the structure obtained from the specific temperature used, and then quenching the steel to transform its remaining austenite to martensite followed by tempering the martensite to substantially the same hardness as the structure produced by the first named temperature without tempering this latter structure. In this 1 manner a hardened steel is obtained having uniform hardness throughout and representing a now accepted as identifying the product forme by the direct decomposition of austenite at temperatures producing neither coarse pearllte nor martensite, along with martensite tempered to the hardness of the bainite produced at the particular temperature used. The microscopic structure of the steel shows a mixture of the dark etching acicular aggregate recognized by modern metallurgists as representing the bainite, along with the well known martensite structure. Microscopic cracks are absent and the steel has a higher tensile strength than bainite while retaining the latter's desirable characteristics to a considerable degree.

As to the specific amount of austenite allowed to transform to bainite, it has been found that as little as 25% prevents the development of microscopic cracks, such as would frequently follow quenching and tempering to an equivalent hardness, and it has also been found that the austenite transformation proceeds at a relatively rapid rate until about has transformed to bainite, it

then requiring in many instances a much longer time. Therefore, the portion transformed should range from about 25 to 75%. Under this range there is a danger of excessive loss in ductility and impact strength for the hardness obtained, and above this range the transformation period becomes impractically'long in certain instances while the gain in ductility and impact strength is in many instances not commensurate with the disadvantages involved by the greatly prolonged transformation period required to effect a transformation of the austenite.

To illustrate the foregoing, tensile specimens were treated to produce the desired mixed structures described in the .case of .180" rounds which were 7" long. The steel contained .78% carbon, .58% manganese, .146% silicon and was otherwise of a composition common to this type of steel. The mixed structures consisting of the described proportions of bainite and tempered martensite were produced by quenching specimens,

heated to an austenitic condition, in a constant tempered bath of 600 F., and holding the specimens therein for the various times required to produce the various mixed structures, the sp cimens then being quenched into brine which caused the remaining austenite to transform to martensite and they then being tempered at 600 F. to a hardness of Rockwell C 50-51 which is the mixtures of the two structures during the austenite transformation at 600 F. are shown in the following Table 1:

Time intervals required to complete successive stages in transformation at 600 F.

Percent bainite Time in seconds Direct brine quench 25 140 50 195 75 275 100 700 It will be noted that to produce from 25 to 75% bainite required only from 140 to 275 seconds, whereas to jump from 75 to 100% bainite required almost triple the amount of time.

The hardness attained by this partial austenite' transformation is illustrated by the following Table 2:

Hardness of bainite-Untempered martensite aggregates Percent Rockwell C ggfig tempered y martensite conversion v The specimens containing martensite were then tempered at 600 F. for a period sufficient to reduce the hardness to that of the full bainite structure. These tempering time intervals are shown in the following Table 3:

Tampering time intervals and resulting hardness values Percent Tempering Rockwell 1 3 untempered time in C b martensite minutes conversion Tensile specimens treated to produce the desired mixed structures as outlined by the forereached, thusfixing 180 as the maximum angle of bend.

aaoaces The results of these mechanical tests are shown by the following Table 4:

Mechanical properties Percent Percent U. 'I. S Impact str. Bend in bainite 5 2 3232 59 in p. s. l in ft. lbs. degrees In further explanation of Table 4, it is to be noted that steel having the new mixed structure with the specified ranges of bainite and martensite, involve relatively smaller loss in ultimate tensile strength as the bainite percentage increases from 25 to 75%, as compared to the loss when the bainite percentage increases from the higher limit of the range up to. 100% bainite.

The particular structure containing 75% untempered bainite and 25% martensite tempered 'to the hardness of the untempered bainite not tendant banding, which steels are very diflicult to process to 100% bainite. Furthermore, the present invention makes it practical to treat steel of larger dimensions than can be practically handled by the prior art treatment previously described. I

I claim:

e 1. A h'ardenablev steel heat treatment including heating the steel to render it austenitic, quenching the steel to a temperature causing its austenite to directly transform to 'a structure other than coarse pearlite or martensite, holding the steel at said temperature until only a portion of its austenite transforms to said structure, quenching the steel to transform its remaining austenite to martensite and tempering the martensite to substantially the same hardness as the structure produced by the first named quenching without tempering this structure.

2. A hardenable steel heat treatment including heating the steel to transform its structure to austenite, quenching the steel under conditions transforming from 25 to 75% of its'austenite to bainite and further quenching under conditions transforming the remainder of its austenite to martensite followed by tempering of this martensite to substantially the same hardness as the bainite without tempering the latter.

. 3. Steel having a mixed structure comprising from 25 to 75% untempered bainite and the remainder tempered martensite of substantiallyv the same hardness as'the untempered bainite. HAROLD J. ELME'NDORF. 

